You point out something really valid here. 15 years, is a long time for any relationship. We can fit this situation into a failing marriage model and all that stuff. Or having your kids stay at home, and not move out on their own. But, more interesting is that maybe staying at one place too long isn't a good thing. It could be a reason, after a certain time, to have students move on and start their own dojos. That should be expected and accepted by everyone in the dojo. That is what I think you should do, start your own dojo.